The use of surface piercing propellers to increase the efficiency of watercraft is known in the industry. Inventor Small adapted such use into a tunnel design, as disclosed in U.S. Pat. Nos. 4,689,026; 6,045,420; 6,193,573; and 6,213,824, all of which are incorporated herein by reference. These patents claim various tunnel configurations for the use of such propellers in shallow draft vessels. Specifically U.S. Pat. No. 6,213,824 teaches a tunnel that raises the propeller vertically to reduce draft. This patent has an inlet ramp or chute that feeds water flow to the propeller when the craft is moving forward on plane.
Surface piercing propellers operate efficiently when a portion of the blade breaks the surface of the water. Shallow draft vessels that employ these propellers housed within a tunnel rely upon a configuration that allows air to be placed in a position directly before the propellers. Through proper tunnel design, the propellers operate as an air pump drawing the air through a conduit. The shape of the tunnel is calculated to provide efficient operation at cruising and/or top speed.
In the teachings of Small, the shape of the tunnel around the surface piercing propeller is just slightly larger in width than the propeller diameter. If the tunnel width is too wide then the ability of the propeller to act like a pump begins to decrease. If the tunnel width is too narrow, inadequate water may lead to excess propeller ventilation. Unique to the tunnel shape of Small is an inlet ramp, or chute, along the leading edge which directs water up to meet the propeller. While the prior art tunnels allow for very efficient vessel operation while on plane, the tunnel design does not provide efficient operation when the vessel is traveling beneath planning speeds or transitioning from off plane to on plane operation. More specifically, the tunnel design of Small fails to provide adequate water flow to the propeller during acceleration.
When forward motion is inadequate for the chute to direct water into the tunnel, the required water must come from in front of and below or in front of and from the sides of the propeller. The current tunnel design inhibits the flow of water during a transition stage from idle to planning, resulting in poor acceleration. The result is known as propeller blow out, or excess propeller slip.
Thus, what is needed is a tunnel configuration that employs the benefits of the surface piercing propellers for shallow draft vessels but addresses the problem of propeller slip.